1. Field of the Invention
The present invention relates to a mobile terminal incorporating a radio communication controller, and in particular, to a mobile terminal utilizing a so-called hand-over function with which a base station is automatically switched so as to continue communications when a user moves from one cell to another.
2. Related Art
Mobile terminals including mobile phones, when located in the cell of a base station, transmit/receive voices and data with the base station using a radio wave, to thereby perform communications with other mobile terminals or fixed telephones via the base station.
Since mobile terminals are small and light, a user can move while speaking. If the moving range of the user is within the cell of a base station, no problem arises. However, the cell of a base station is invisible, so that there may be a case that the user moves over the cell of a base station into the cell of another base station. In such a case, the communicating condition between the mobile terminal and a base station becomes better with the destination base station than with the source base station. Therefore, it is necessary to change a base station, with which the mobile terminal communicates, to the destination base station. A mobile terminal utilizes a so-called hand-over function with which a base station is switched automatically when a user moves from one cell to another so as to continue communications.
When a new service is provided from a base station to a mobile terminal, a radio wave for the new service uses a higher frequency in order to avoid the existing frequency band. As higher frequency is used, the attenuation factor of the radio wave becomes higher. As such, the cell region of the base station tends to be smaller. This means that as more new services are provided, the number of hand-over increases.
A mobile terminal with a PDC(personal digital cellular) system adopts FDD(frequency division duplication) in which a transmission frequency band and a receiving frequency band are separately existed. Each frequency band is divided into three, and one of them is used as a time slot of the mobile terminal itself. Operations of a mobile terminal include processing of a transmission time slot, processing of a receiving time slot, and processing of time slots other than these. In a time slot where the mobile terminal does not perform transmission/reception, the mobile terminal monitors the strengths of radio waves from other base stations with which no communicating state is established.
If certain condition values are indicated in the radio wave strength from a base station in the communicating state and in the radio wave strength from a base station not in the communicating state, the base station is switched upon request from the mobile terminal.
In a W-CDMA(Wideband CDMA) system, although each base station of the same service provider performs communications with a mobile terminal using the same frequency, each base station is assigned a different scramble code. As obvious from the fact that a control signal for such as starting a measurement is transmitted from the base station to a mobile terminal (downward direction), the base station holds an initiative to control a hand-over. However, actual measurements and announcements to the base station are performed on the mobile terminal side. FIG. 2 shows a sequence, which extracted and translated FIG. 26 of the technical specification TS25.303 of W-CDMA. As seen from this FIG., it is not necessary to reply all measurement results obtained by the terminal as they are. The measurement results are notified from the mobile terminal to the base station according to the independent criteria of the mobile terminal.
Even in either system, there are following problems since all cells of other base stations (destination), adjacent to the cell of the base station (source) (service area cell) with which the mobile terminal is currently connected, are subject to monitoring as hand-over targets.
First, when a terminal side such as PDC monitors surrounding cells, all surrounding cells are subject to monitoring. Therefore, cells of base stations, into which the user has never moved before, are also subject to monitoring. Monitoring of these cells is often just a waste of electric power, which only causes a shortening of the movable time of the mobile terminal.
Second, as the number of monitoring targets increases, the period of time assignable to one monitoring target decreases. Therefore, it is forced to measure in a short period of time, so that a search accuracy is degraded. In particular, since a search cycle is shorter and a radio wave with a higher frequency is used, comparing with those of the PDC system, this problem is remarkable in the W-CDMA system in which a micro-cell or a pico-cell is used as a cell and the number of search targets tends to increase.
In order to solve these problems, there are conventional techniques as described below.
In the technique disclosed in the Japanese Patent Application Laid-open No. 11-075237, a mobile terminal makes a preliminary selection continuously for an adjacent cell satisfying a certain condition. When a hand-over or the like occurs since the strength of the electric field is lowered, the adjacent cell which is a target of the preliminary selection becomes a target of monitoring.
A technique disclosed in the Japanese Patent Application Laid-open No. 2000-209630 applies a different cell search algorithm depending on whether the mobile terminal is in the moving state or in the still state. When in the still state, a base station which has been stored as a hand-over destination by the mobile terminal is set as a target to be preferentially searched, and cell searches are performed to the base station with high frequency than to other base stations. On the other hand, in the moving state, a base station which is adjacent to the base station currently connected is set as a target to be preferentially searched for, and cell searches are performed to the base station with high frequency than to other base stations.
In either technique, an absolute element such as a physical distance is set as the basis for cell searches, as described above. However, this basis is not a basis in which information such as an activity range of a user is reflected. More specifically, in the technique disclosed in the Japanese Patent Application Laid-open No. 11-075237, a basis for determination is a quality of service provided from surrounding base stations by information elements included in a system information message (how many cells are available for GPRS preferable for data transfer). Further, in the technique disclosed in the Japanese Patent Application Laid-open No. 2000-209630, a mobile terminal preferentially sets a cell, which is determined as appropriate by the base station, as a target of a search.